Glycogen synthase kinase 3 (GSK3) is a family of serine/threonine protein kinases that consists of two isoforms, GSK3a and GSK3. In the kidney, GSK3 is known to regulate cell differentiation and normal epithelial function. Despite in vitro and in vivo evidence that lithium, a common therapy for bipolar disorders and a potent inhibitor of GSK3 can reduce renal response to vasopressin, the role of GSK3 in renal water transport is not clear. Our observations in the lithium induced polyuric mouse model as well as collecting duct specific GSK3 knockout mice showed that GSK3 plays a significant role in renal water reabsorption by regulating aquaporin 2 expression in response to vasopressin. In the GSK3 knockout mice, adenylate cyclase activity and cAMP levels were reduced by an undetermined mechanism, which could have led to low aquaporin 2 expression and trafficking in response to vasopressin. Based on these evidences we hypothesize that GSK3 plays a critical role in renal water homeostasis. The role of this kinase in normal renal water reabsorption will be determined in the following 3 aims. 1) The first aim will determine how GSK3 regulates adenylate cyclase activity by examining if GSK3 binds to or phosphorylates adenylate cyclase using site directed mutagenesis. 2) The mechanism by which AVP signaling activates GSK3 will be determined by testing the hypothesis that AVP signaling activates GSK3 by inhibiting the canonical Wnt signaling. A negative feedback loop by which protein kinase A might regulate GSK3 will also be examined. 3) The third aim will test the hypothesis that AVP resistant- lithium induced NDI is a pathophysiological consequence of inhibition of renal GSK3. Since increased AVP signaling and high cAMP levels contribute to the progression of polycystic kidney disease, this aim will also test the hypothesis that inhibition or gene deletion of GSK3 can reduce cystogenesis. These studies will utilize wild type and collecting duct specific GSK3 knockout mice, primary cultures of inner medullary collecting duct cells and mouse cortical collecting duct cells. Through these studies we exoect to identify the mechanism by which GSK3 regulates AVP signaling in the renal collecting duct and its physiological and pathophysiological significance.